1. Field of the Invention
This invention relates to a limited-displacement device such as a galvanometer whose displacement corresponds with the magnitude of an electric signal supplied thereto. More specifically, the invention relates to the use of frequency and phase compensation to provide a galvanometer with an extended frequency range.
2. Prior Art
Limited-rotation motors are used extensively in instruments which rotate to an angular position that corresponds with an input voltage or current. The motor may be used to deflect a light beam by rotating a mirror or a light source or it may move a recording pen in a strip chart recorder that records the value of a monitored parameter as a function of time. These motors, together with the components rotated by them, are generally called galvanometers. As used herein the term "galvanometer" also includes other limited-displacement devices which undergo reciprocal rotational or linear motion in response to an applied signal. These devices include, for example, optical scanners which reciprocate mirrors in response to predetermined input signals.
A galvanometer usually comprises, in addition to the motor, a spring that urges the motor into a neutral position and against which the motor must rotate in order to displace an output element. The motor is usually an electromagnetic transducer whose torque is proportional to the electric current passed through it and the spring is linear, i.e., its torque is proportional to its angular displacement. The displacement is therefore proportional to the current. If an output displacement proportional to an applied voltage is desired, the system includes circuitry that provides a motor current proportional to the applied voltage.
However, the various components of a galvanometer, primarily the spring and the moving mass, form a resonant system and the proportionality between the output displacement and the magnitude of the input signal therefore varies with frequency. Indeed, the displacement drops off sharply at frequencies greater than the resonant frequency of the system. This can limit the response of the galvanometer to relative low frequencies, e.g., 50 Hz or less.
One can pass the input signal through a compensating network to increase the displacement at frequencies above the resonant frequency. However, a circuit that compensates the displacement in this manner will not, in general, compensate for the varying phase characteristic of the output. Accordingly, there is a substantial variation of phase as a function of the frequency of the input signal and this is especially significant in closed-loop systems in which the output displacement is compared with an input signal and the resulting error signal is used to drive the motor. Specifically, the phase deviation can cause instability unless the gain and bandwidth are severely limited.